Demonstrate working of two stroke S.I. engines, four stroke SI engine and C.I. engines.

AIM : Demonstrate working of two stroke S.I. engines, four stroke SI engine and C.I. engines.

2- Stroke Engine:

The two stroke engine is a reciprocating engine in which the piston does two time movement ( i.e. TDC to BDC and BDC to TDC ) to produce a power stroke

Upward stroke:

• During upward stroke, the piston moves from BDC to TDC and compresses the charge (air-fuel mixture) in the combustion chamber of the cylinder.
• Because of the upward movement of the piston a partial vacuum is created in the crankcase and this allows the entry of the fresh charge into the crankcase through uncovered inlet port.
• The exhaust port and the inlet port remains covered when the piston at the TDC.
• The ignition of the fresh charge is takes place by the spark plug.

Downward stroke:

• As soon as the combustion of the fresh charge takes place, a large amount of the hot gases is produced which exerts a very high pressure force on the top of the piston. Due to this high pressure force, the piston moves downward and rotates the crankshaft and does useful work.
• During this stroke the inlet port is covered by the piston and the new charge is compressed in the crankcase.
• Further downward movement of the piston uncovers first the exhaust port and the transfer port and the exhaust starts through the exhaust port.
• As soon as the transfer port opens, the charge through it is forced into the cylinder.
• The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out most of the exhaust gases.
• The piston is now at BDC position. The cylinder is completely filled with the fresh charge but it is somewhat diluted with the exhaust gases.
• Finally the cycle event is then repeated. We get two strokes for the single revolution of the crankshaft.

Four Stroke Petrol Engine:

The cycle of operation in a four stroke petrol engine is completed in two revolutions of crank shaft or four strokes of piston. Stroke is defined as the distance traveled by the piston from one of the dead centers to the other dead centre. It is also equal to two times the crank radius. Hence in a four stroke engine work is obtained only during one stroke out of the four strokes of the piston required to complete one cycle. This engine works on Otto or constant volume cycle.

1. Suction stroke: To start with the piston is at or very near T.D.C. and the inlet valve is open and exhaust valve is closed. As the piston moves from T.D.C. to B.D.C. rarefaction is formed in the cylinder which causes the charge to rush in and fill the space vacated by the piston. The charge consists of a mixture of air and petrol prepared by the carburetor. The admission of charge inside the engine cylinder continues until the inlet valve closes at B.D.C.

2. Compression stroke: Both the valves are closed and the piston moves from B.D.C. to T.D.C. The charge is compressed up to a compression ratio of 5:1 to 9:1 and pressure and temperature at the end of compression are about 6 to 12 bar and 250º C to 300º C respectively.

3. Working, Power or Expansion stroke: When the piston reaches T.D.C. position, or just at the end of compression stroke, the charge is ignited by causing an electric spark between the electrodes of a spark plug, which is located some where in the walls of cylinder head. During combustion the chemical energy of fuel is released and there is rise in temperature and pressure of gases. The temperature of gases increases to about 1800º C to 2000º C and the pressure reaches 30 to 40 bar. Up till now the volume of gases formed however remains almost constant with both valves closed. Now the combustion products expand and push the piston down the cylinder. The reciprocating piston motion is converted into rotary motion of crankshaft by a connecting rod and crank. During expansion the pressure drops due to increase in the volume of gases and absorption of heat by cylinder walls.

4. Exhaust stroke: Theoretically exhaust valve opens at the end of working stroke when the piston is at B.D.C. position. But actually exhaust valve begins to open when about 85 % of the working stroke is completed. A pressure of 4 to 5 bar at this instant forces about 60 % of the burnt gases into the exhaust manifold at high speed. The remaining burnt gases are cleared off the swept volume when the piston moves from B.D.C. to T.D.C. During this stroke the pressure in side the cylinder is slightly above the atmospheric value. Some of the burnt gases are however left in the clearance space. The exhaust valve closes shortly after the piston reaches T.D.C. The inlet valve opens slightly before the end of exhaust stroke and the cycle repeats. 

Four Stroke Diesel Engine:

The cycle of operation in a four stroke diesel engine is completed in two revolutions of crankshaft or four strokes of piston using diesel oil as fuel. This engine works on diesel cycle.

1. Suction Stroke: Starting of engine is done by an electric motor or manually. In both cases the energy is supplied to the engine. In this stroke the inlet valve opens and the outlet valve remains closed. Piston moves from T.D.C. to B.D.C. and in this way a vacuum is created in the cylinder. This vacuum is filled by air alone and piston reaches to B.D.C.

2. Compression Stroke: Both valves are closed. This time piston moves from B.D.C. to T.D.C. Air is compressed in this stroke up to a compression ratio of 15:1 to 22:1 and a very high temperature is produced due to high pressure. The high temperature is the only cause of combustion of the fuel. The piston takes the power in this stroke from the flywheel. During this stroke the pressure and temperature attain a high value of 40 to 60 bar and 600º C to 700º C.

3. Working Stroke: At the end of compression stroke or when the piston reaches the T.D.C. position, a fine spray of diesel is injected in the cylinder through injector. The fuel burns by the heat of compressed air and due to its burning the power is produced. This power pushes the piston downward i.e. from T.D.C. to B.D.C. The excess energy of the piston is stored in the flywheel of the engine, which is further used for the remaining three strokes of the engine. The reciprocating motion of the piston is converted into the rotary motion of the crankshaft by connecting rod and crank. During expansion the pressure drops due to increase in volume of gases and absorption of heat by cylinder walls.

4. Exhaust Stroke: The exhaust valve begins to open when about 85% of the working stroke is completed. The force of piston coming from B.D.C. to T.D.C. forces the burnt gases into the exhaust manifold. Some of the gases are forced out due to higher pressure in the cylinder and the remaining gases are forced out by the piston. Some of the burnt gases are however left inside the clearance space. The exhaust valve closes shortly after T.D.C. The inlet valve opens slightly before the end of exhaust and in this way the cycle repeats.